hydride

The position of an element in the periodic table
is a good guide to the type of hydride it will form. The alkali and
alkaline earth metals (on the left of the table above) form ionic
hydrides in which hydrogen is the acid radical. The non-metals also
gives rise to ionic hydrides, but in these hydrogen is present as
a positive ion which can be replaced by metallic ions.

A binary compound of hydrogen with another,
more electropositive element or group (one that loses electrons and forms
positive ions). Hydride ions and hydride ionic compounds react instantly
and sometimes violently with water.

There are several different types of hydrides.

Salt-like. These are the hydrides of the most electropositive
elements (e.g. sodium) and contain ions.
They are crystalline solids and react violently with water to give hydrogen.

Covalent. These are formed by most of the non-metals and transition
metals, in groups IB–VIIA of the periodic table. They are
mainly volatile, reactive compounds, though those of groups IB and IIB,
and aluminum, are nonvolatile polymers. Covalent hydrides include such
diverse compounds as methane and iron
carbonyl hydride, H2Fe(CO)4. In many compounds
the hydrogen atoms act as a bridges. Where there are more than one hydride
sites there is often hydrogen exchange between the sites.

Complexes. These derivatives contain complex anions which may
be considered as derived from co-ordination of an H- ion
to a metal or non-metal. Examples are the BH4-
and ReH92- ions.

Transition metal hydrides. These are formed by hydrogen uptake
by transition elements (groups IIIB–VIII). The phases are often
non-stoichiometric and electrically conducting. They resemble alloys
and some have interstitial structures.

Negatively-charged hydrogen ions

When a direct current is passed through dilute hydrochloric
acid, hydrogen gas can usually be collected above the cathode.
The dilute hydrochloric acid contains hydrogen ions and this experiment
shows that the hydrogen ions carry positive charges and under the influence
of the electric current they migrate toward the electrode with the opposite
(negative) charge.

However, compounds containing hydrogen do not necessarily yield positively
charged hydrogen ions. If an electric current is passed through molten lithium
hydride (LiH), the lithium ions travel toward the cathode while
hydrogen gas can be collected above the anode. In this instance
the hydrogen ions are negatively charged. This unusual property
is characteristic of the compounds which hydrogen forms with the alkali
and alkaline earth metals (lithium, sodium, potassium, calcium, etc.).

Hydrogen combines with many elements to form compounds called hydrides.
The compounds of hydrogen with the more common non-metallic element such
as sulfur, chlorine, and iodine are usually regarded as hydrides because
most of them yield acid solutions when they are dissolved in water. They
give rise to series of metallic salts such as the sulfides and iodides.
However, the term hydride (which should not be confused with hydrates,
meaning compounds containing chemically combined water) can be applied to
any compound containing hydrogen and one other element.

The hydrides can be divided into several groups according to their properties.
These groups correspond quite closely to the position of the other element
in the periodic table. The alkali and alkaline earth metals and the halogens
(non-metals) are at opposite ends of the horizontal rows or periods
in the table, and the hydrides which they form behave in quite different
ways. But all these hydrides are ionic – the non-metallic ones give
positively charged hydrogen ions, while the metallic ones yield negatively
charged hydride ions. But between these extremes there are many more hydrides
both of metals and non-metals. The parts of these molecules are held together
by much stronger forces than the in the ionic hydrides and no ions are formed.

Ionic metal hydrides

All the hydrides of the alkali and alkaline earth metals are strong reducing
agents. They react with water, some of them violently. Hydrogen gas
is liberated and the hydroxide of the metal is formed. The reaction between
water and calcium hydride is, by comparison, quite mild. For this reason
calcium hydride is used as a convenient source of portable hydrogen, just
as calcium carbide is used for making acetylene. Both lithium hydride and
lithium aluminum hydride (liAlH4) are very useful for producing
nascent hydrogen when preparing organic substances.

The hydrides of this group are fairly stable, colorless, crystalline compounds
which resemble, in some ways, the chlorides and iodides of the same metals.
They are usually prepared by passing hydrogen under pressure over the heated
metal.

Other metal hydrides

Although it is doubtful whether true chemical compounds are formed between
hydrogen and some metals, it is certainly true that hydrogen gas is absorbed
by many metals when they are heated in the gas. The power of the metal palladium
to absorb hydrogen is outstanding. Powdered palladium which has been kept
in an atmosphere of hydrogen at 100°C for three hours, and then left
to cool in the gas for 90 minutes absorbs as much as 650 times its own volume
of the gas.

Non-metallic hydrides

Ionic hydrides of non-metals

Hydrogen sulfide is an important reagent which is
used in qualitative analysis

This group includes compounds such as hydrogen
chloride, hydrogen iodide, hydrogen
sulfide, and water. Some of these compounds
(e.g. hydrogen iodide and water) can be prepared by direct combination between
the elements concerned. However, hydrogen chloride and hydrogen sulfide
can be obtained much more easily by the action of a strong acid (not nitric
acid) on an appropriate metallic salt. For instance, hydrogen chloride is
generally prepared by heating sodium chloride and concentrated sulfuric
acid.

When these hydrides are dissolved in water they form acid solutions and
positively charged hydrogen ions are produced. These hydrides give rise
to well-defined series of salts – chloride,
iodides, and sulfides.

The hydrides of oxygen (water and hydrogen
peroxide) do not appear to resemble any of the other hydrides very closely.
Water occupies an almost unique position – many of its unusual properties
are due to hydrogen bonding between
molecules.

Other non-metal hydrides

The hydrides of carbon (the hydrocarbons)
are probably the most important hydrides of this group. This almost limitless
range of compounds is present in crude oil, or is produced at various stages
in refining. The hydrocarbons are valuable as fuels and, even more important,
as the raw materials used in the manufacture of a vast range of substances
including plastics and artificial fibers.

There are aliphatic (or chain)
hydrocarbons in which hydrogen atoms are attached to a chain of carbon atoms,
while the basic unit of the aromatic
(ring) hydrocarbons is a ring of six carbon atoms with one hydrogen atom
attached to each.

Both silicon and boron
resemble carbon to some extent, but the ability to form chain hydrides is
rather limited. The chains may contain up to six atoms per molecule. Compounds
of the aromatic type do not exist. The hydrides of boron and silicon are
by no means so stable as those of carbon.

The hydrides of two other non-metals – nitrogen
and phosphorus – are also important
and have certain properties in common. Both ammonia
(NH3) and phosphine (PH3) are alkaline and form halides,
though in these respects phosphine is much weaker. Ammonia actually gives
rise to an ion – the ammonium ion – which behaves in a similar
way to the ions of alkali metals.